This invention relates to an improvement of monostable multivibrators. More specifically, it relates to a monostable multivibrator that stabilizes the output pulse width when a low-pass filter is connected to the output as in the case of using it in a pulse count system FM demodulator.
As shown in FIG. 1, a conventional monostable multivibrator may comprise a gate circuit 1 such as a NAND gate, a differentiator time constant circuit 4 including a capacitor 2 and a resistor 3, and a differential inverter comprising a current switching circuit where the common emitters of transistors 5 and 6 are connected to a constant current source 7. The collector of transistor 5 is grounded through a load resistor 8 while the collector of transistor 6 is connected to the -B power supply through a load resistor 9. The base of transistor 5 has applied thereto the output of differentiator circuit 4 while the base of transistor 6 has applied thereto the +B power supply as divided by resistors 10 and 11. An output is obtained from the collector of transistor 6 by applying a trigger pulse to one of the input terminals of gate circuit 1, the collector output of transistor 5 being applied to the other input terminal of the gate circuit.
Regarding the differential inverter circuit, changes in the ambient temperature with respect to the respective emitter-base voltages, V.sub.BE, of transistors 5 and 6 identically affect both transistors. Also the current of transistor 6 is regulated by constant current source 7 so that the carrier accumulation time is shortened. Thus, carrier accumulation time fluctuations with respect to changes in the ambient temperature can be virtually ignored. Hence, fluctuations in the output pulse width caused by changes in the ambient temperature of this monostable multivibrator are reduced.
On the other hand, as is well-known, a pulse count system demodulator applies the intermediate frequency signal of an FM receiver to a limiter amplifier so that square waves are formed by the limiter amplifier. The output of the limiter amplifier is applied to a differentiator circuit to obtain trigger pulses with narrow pulse width which are synchronized with the rise and fall of the square waves. The output of the differential circuit triggers a monostable multivibrator to obtain output pulses of constant width. The monostable multivibrator output is then applied to a low-pass filter--that is, an integrator which equalizes the output of the monostable multivibrator, to thereby demodulate the intermediate frequency signal to obtain the audio output.
If a low-pass filter is connected to the output terminal OUT of FIG. 1 as in the case of a pulse count system demodulator, the differential inverter and the low-pass filter are as shown in FIG. 2 where 12 is the low-pass filter. When the output frequency of differentiator circuit 4 is high and low-pass filter 12 is not connected, the output of the monostable multivibrator--that is, the collector output waves of transistor 6, is a square wave shown in FIG. 3(a). However, when low-pass filter 12 is connected, the collector output waveform of transistor 6 becomes triangular as also shown in FIG. 3(a). The collector average is V.sub.C1 as shown by the phantom line in FIG. 3(a).
Further, when the output frequency of differentiator circuit 4 is low and low-pass filter 12 is not connected, the collector output waveform of transistor 6 is a square wave as shown in FIG. 3(b). When the low-pass filter 12 is connected, the collector output waveform of transistor 6 becomes triangular as shown in FIG. 3(b), the mean voltage being V.sub.C2 as shown by the phantom line in FIG. 3(b). In other words, the average collector voltage V.sub.C of transistor 6 fluctuates with the output frequency of differential circuit 4.
Generally, when an inverse voltage V.sub.BC is impressed on the collector-base of a transistor, the collector-base capacitance C.sub.BC changes with the inverse voltage. Therefore, when (a) low-pass filter 12 is connected to the output terminal as shown in FIG. 2, and (b) the base voltage V.sub.B of transistor 6 is constant, and (c) the average collector voltage V.sub.C of transistor 6 fluctuates with the output frequency of the differentiator circuit 4 as described above, the collector-base inverse voltage V.sub.BC of the transistor 6 fluctuates so that the collector-base capacitance C.sub.BC changes. Thus, the FIG. 2 arrangement has shortcomings in that the amount of negative feedback of the differential inverter circuit changes, the gain changes, and the output pulse width of the monostable multivibrator fluctuates. Further, when this monostable multivibrator is used in a pulse count system demodulator, the fluctuation of the output pulse width appears as distortion when the FM signals are demodulated. If a low-pass filter that increases the load capacitance sufficiently with respect to C.sub.BC is used in order to eliminate these problems, the changes in C.sub.BC can be ignored and the changes in the amount of negative feedback can also be ignored, thus eliminating the above problems. However, this has the shortcoming of narrowing the reproduction zone of the demodulator.